All About Glow Plug

HOW TO CHOOSE THE RIGHT GLOW PLUG

For example : The O'DONNELL 99T is colder than the 97T, and is more intended for the extended high RPM running that on-road engines generally see (generally more than off road, unless your track has many long straight sections).
The O'Donnell site and Amain descriptions are correct, just worded differently. Amain calls out the heat range of the plug, OD calls out the conditions that it's intended for.
Of course, nitro content, tuning preference, engine specifics, and personal choice are factors, but a rule of thumb is this...

Hotter weather = cooler plug needed

Cooler weather = hotter plug needed

Smaller engine = hotter plug needed

Higher nitro content = colder plug needed

The "right" glow plug for your engine is the one that gives you the best performance. And you can choose the right plug for any situation, just by following the guidelines below.

1. Engine Type

Guideline 1: Know what type of engine you have. Is it a standard - or a turbo?

Standard engines (engines with a 1-piece head) are most common. Standard plugs are easily available, inexpensive and fit almost all standard engines. Standard plugs are installed with a washer, which creates a compression seal with the head.
Many new O.S. engines are turbo engines, which feature a special 2-piece turbo head. The biggest benefit of turbo plugs is superior performance. Unlike standard plugs, turbo plugs (identified by a "P" in the description) feature a tapered "seat" that matches perfectly with the head. That creates a superior compression seal and with it, maximum efficiency and power. Turbo plugs are the choice for racers who want - and need - top performance.
A word of caution: you should never install a turbo plug in a standard engine or vice versa. Doing so risks doing serious (and expensive!) damage.

2. Displacement

Size matters to glow plugs. What size is your engine? A .12? .15? .21? Big engines have more mass and retain heat better. Smaller, lighter engines don't, and need the help a hotter plug can offer.

Guideline 2: The smaller the engine, the hotter the plug.

3. Fuel Nitromethane Content

What's the nitro percentage in your fuel? High-nitro fuels produce more power than low-nitro fuels, but also produce more heat.

Guideline 3: The higher the nitro content, the colder the plug.

4. Temperature

Smart modelers tend to keep a variety of glow plugs on hand. The reason? Because the "right" plug for your engine can change with the temperature. To achieve top performance, your choice of plug needs to change, too.

Guideline 4: The hotter the day, the colder the plug.

5, 6 & 7. Other Considerations

Here are a few other things you should know.
Hot plugs promote better idling and acceleration. If your engine runs rough or accelerates sluggishly, a hotter plug will help.
Cold plugs produce more power and may improve performance if your engine runs hot. The downside is rougher idling and more difficulty in tuning.
Where you run also plays a part. If the track/course has a lot of twists and turns, a hot plug is fine. If the track/course has long straights where you'll reach maximum rpm, a colder plug is best.

Fuel-air mix not only affects how your engine performs; it can also have an impact on how long your plug lasts. If you run rich, it means that you're using more fuel than necessary for top performance. Modelers are often advised to run rich during engine break-in, because it helps cool the engine. However, running too rich can also cause an engine to "bog down" or quit entirely. In addition, it also means that the glow element is being exposed to more contaminants than necessary, which shortens plug life.

Running lean means that you're using less fuel. "Leaning down" an engine has a positive effect on performance. However, care is needed here, because over-leaning an engine can harm it, by raising operating temperatures, "burn up" a plug before its time.

Final Thoughts

Choosing the right glow plug not only improves performance, but can also extend the life of your engine and the glow plug itself. With the guidelines above and the tips below, you're well on your way to achieving both.